An aluminum alloy is lightweight and has high thermal conductivity, and thus is used in a heat exchanger for an automobile, for example, a radiator, a condenser, an evaporator, a heater or an intercooler. The heat exchanger for the automobile is mainly produced via brazing. In general, brazing is conducted at a high temperature of about 600° C., by using an Al—Si-based filler alloy.
An aluminum alloy heat exchanger to be produced through brazing is formed of a corrugated fin for mainly conducting heat radiation and a tube for circulating cooling water or refrigerant. In the case where the tube is broken and pierced, the cooling water or refrigerant inside the tube leaks. Thus, an aluminum alloy brazing sheet having excellent strength after brazing is essential for extending a life of a product using the same.
In recent years, a demand for reduction in weight of an automobile has been increasing, and corresponding reduction in weight of an automobile heat exchanger has been required. Thus, reduction in thickness of each member forming the heat exchanger has been studied, since there is a need of an aluminum ally brazing sheet having further improved strength after brazing.
Hitherto, a tube material of a heat exchanger in which cooling water circulates inside the tube, such as a radiator or a heater for an automobile, generally employs a three layer tube material obtained by: cladding a sacrificial anode material, such as Al—Zn-based alloy, on an inner surface of a core alloy, such as an Al—Mn-based alloy typified by JIS 3003 alloy; and cladding a filler alloy, such as Al—Si-based alloy, on an atmospheric side of the core alloy. However, the mechanical strength after brazing of the clad material employing the JIS 30003 core alloy is about 110 MPa (110 N/mm2), which is insufficient.
It has been proposed that coarsening of precipitated grain is suppressed to thereby improve mechanical strength of an aluminum alloy brazing sheet by suitably regulating conditions for homogenization treatment of a core alloy and by keeping the core alloy at the temperature of less than 500° C. before hot rolling (JP-A-8-246117). However, in this producing process, a beginning temperature and an ending temperature for hot rolling cannot be taken into consideration, and there is a possibility that the intermetallic compounds, which exert a negative influence on an aging hardening effect based on Mg2Si after brazing, is precipitated, resulting in a problem that the aluminum alloy brazing sheet can not have a sufficient strength after brazing.
In order to satisfy demands for reduction in thickness of an aluminum alloy brazing sheet, properties such as strength after brazing must be improved. However, conventional techniques have difficulties in assuring properties with a small thickness, while attaining higher strength simultaneously.